Optical filters are important components of optical devices dealing with multi-wavelength light signals. Such filters, such as Bragg gratings, Fabry-Perot etalons, etc, often present a comb-like spectral response, i.e. their reflectivity or transmission characteristics have a certain periodicity as a function of wavelength. Fiber Bragg Gratings are one type of such filters which are widely used in a lot of applications of photonics, and more particularly in optical telecommunications devices. In a lot of these applications the spectral response of two or more FBGs, either single or multi-channel, must be precisely known, that is, precisely aligned one with respect to another, and with absolute spectral position if possible. To obtain such an alignment and calibration, spectrally-resolved equipment is generally required, such optical spectrum analyzers (OSA) or other calibration apparatuses. Access to such equipment is however not always forthcoming and their use in the field is both costly and unpractical.
There is a need to devise a method to perform such an operation, simply and at low cost, without requiring optical spectrum analyzers or other calibration apparatuses.
A particular application where such a precise spectral alignment is needed is in the case of dispersion compensators, FBG-based tunable optical devices in which two chirped gratings are sometimes stretched with respect to one another to generate a combined specific optical response at given specific wavelengths (that can be continuous along the optical band). A non-restrictive example of such a dispersion compensator is shown in Applicant's own U.S. Pat. No. 6,937,793 (LELIÈVRE et al), which is incorporated herein by reference.
Given the high wavelength dependency of such a device, a precise calibration of the spectral response of both gratings is needed in order to precisely position them and get the expected spectral response and dispersion level. This calibration should be easy to obtain again at any point during the lifetime of the device, should the spectral positions of the grating drift over time or be lost after a malfunction of the system it is part of. This should be done without resorting to a spectral analysis, as wavelength-specific optical monitoring is not always available (nor wanted) in practice during this operation.
There is therefore a need for an absolute calibration of both gratings spectral position.